linux-kernel - Re: [PATCH] arc: use little endian accesses

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Message-ID: <56E398F1.7090500@synopsys.com>
Date:	Sat, 12 Mar 2016 09:50:01 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Arnd Bergmann <arnd@...db.de>,
	Lada Trimasova <Lada.Trimasova@...opsys.com>,
	"noamc@...hip.com" <noamc@...hip.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
	"linux-snps-arc@...ts.infradead.org" 
	<linux-snps-arc@...ts.infradead.org>
Subject: Re: [PATCH] arc: use little endian accesses

On Friday 11 March 2016 06:14 PM, Vineet Gupta wrote:
> @Lada I will fix up the changelog to add some of the background behind this
> change, and mark this for stable backport as well.

This is what I'm planning to add.
Noam please give this a spin - you might have to revert those native-endian DT
bindings from UART DT.

----->
>From f778cc65717687a3d3f26dd21bef62cd059f1b8b Mon Sep 17 00:00:00 2001
From: Lada Trimasova <ltrimas@...opsys.com>
Date: Wed, 9 Mar 2016 20:21:04 +0300
Subject: [PATCH] ARC: [BE] readl()/writel() to work in Big Endian CPU
 configuration

read{l,w}() write{l,w}() primitives should use le{16,32}_to_cpu() and
cpu_to_le{16,32}() respectively to ensure device registers are read
correctly in Big Endian CPU configuration.

Per Arnd Bergmann
| Most drivers using readl() or readl_relaxed() expect those to perform byte
| swaps on big-endian architectures, as the registers tend to be fixed endian

This was needed for getting UART to work correctly on a Big Endian ARC.

The ARC accessors originally were fine, and the bug got introduced
inadventently by commit b8a033023994 ("ARCv2: barriers")

Fixes: b8a033023994 ("ARCv2: barriers")
Link: http://lkml.kernel.org/r/201603100845.30602.arnd@arndb.de
Cc: Alexey Brodkin <abrodkin@...opsys.com>
Cc: stable@...r.kernel.org  [4.2+]
Cc: Arnd Bergmann <arnd@...db.de>
Signed-off-by: Lada Trimasova <ltrimas@...opsys.com>
[vgupta: beefed up changelog, added Fixes/stable tags]
Signed-off-by: Vineet Gupta <vgupta@...opsys.com>
---
 arch/arc/include/asm/io.h | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/arch/arc/include/asm/io.h b/arch/arc/include/asm/io.h
index 694ece8a0243..27b17adea50d 100644
--- a/arch/arc/include/asm/io.h
+++ b/arch/arc/include/asm/io.h
@@ -129,15 +129,23 @@ static inline void __raw_writel(u32 w, volatile void __iomem
*addr)
 #define writel(v,c)		({ __iowmb(); writel_relaxed(v,c); })

 /*
- * Relaxed API for drivers which can handle any ordering themselves
+ * Relaxed API for drivers which can handle barrier ordering themselves
+ *
+ * Also these are defined to perform little endian accesses.
+ * To provide the typical device register semantics of fixed endian,
+ * swap the byte order for Big Endian
+ *
+ * http://lkml.kernel.org/r/201603100845.30602.arnd@arndb.de
  */
 #define readb_relaxed(c)	__raw_readb(c)
-#define readw_relaxed(c)	__raw_readw(c)
-#define readl_relaxed(c)	__raw_readl(c)
+#define readw_relaxed(c) ({ u16 __r = le16_to_cpu((__force __le16) \
+					__raw_readw(c)); __r; })
+#define readl_relaxed(c) ({ u32 __r = le32_to_cpu((__force __le32) \
+					__raw_readl(c)); __r; })

 #define writeb_relaxed(v,c)	__raw_writeb(v,c)
-#define writew_relaxed(v,c)	__raw_writew(v,c)
-#define writel_relaxed(v,c)	__raw_writel(v,c)
+#define writew_relaxed(v,c)	__raw_writew((__force u16) cpu_to_le16(v),c)
+#define writel_relaxed(v,c)	__raw_writel((__force u32) cpu_to_le32(v),c)

 #include <asm-generic/io.h>

-- 
2.5.0